1. Field of the Invention
The invention relates to a process for preparing isocyanate-terminated siloxanes.
2. Description of the Related Art
Isocyanate-terminated siloxanes are valuable starting materials since they combine the properties of siloxanes with the high reactivity of isocyanates and are thus suitable for producing silicone-containing polyurethanes or polyureas.
The current state of the art describes a variety of methods of preparing isocyanate-terminated siloxanes, as indicated in the following references:
U.S. Pat. No. 3,170,891 describes the preparation of isocyanate-terminated siloxanes by hydrosilylation of unsaturated isocyanates by means of Si—H-terminated siloxanes. However, a disadvantage is the high raw materials costs resulting from the use of the Si—H-siloxanes and also the platinum catalyst required. In addition, the reaction is not quantitative and is characterized by the formation of numerous by-products.
U.S. Pat. No. 3,179,622 describes the synthesis of amine- or carbinol-functionalized siloxanes and diisocyanates. However, the isocyanate-terminated siloxanes obtained in this way are not completely free of the toxic monomeric diisocyanates. In addition, these siloxanes also have urea or urethane units in the chain and these have an adverse effect on the viscosity and storage stability of these materials.
U.S. Pat. No. 5,886,205 describes the preparation of isocyanate-terminated siloxanes by pyrolysis of urethane-siloxanes which have been obtained by reaction of amine-functionalized siloxanes with diorgano carbonates. Owing to the relatively drastic reaction conditions, this synthesis leads to a series of by-products. JP 2001-48855 is concerned with this synthesis in modified form. Although the use of diaryl carbonates results in high yields of isocyanate-terminated siloxanes, the reaction times are very long and high molecular weight siloxanes can be obtained only with great difficulty by this method.
JP 2001-48855 is concerned with this synthesis in modified form. Although the use of diaryl carbonates results in high yields of isocyanate-terminated siloxanes, the reaction times are very long and high molecular weight siloxanes can be obtained only with great difficulty by this method.
A direct synthesis of the isocyanate-terminated siloxanes by means of phosgenation of corresponding amine-functionalized siloxanes is not possible because of the HCl liberated in the reaction. According to U.S. Pat. No. 5,886,205, this strong acid leads to degradation of the siloxane chain during the reaction. Scavenging of the HCl by means of tertiary amines as bases, as is described in the Japanese publication 5-5713 (1993), is not practicable on an industrial scale.
EP 1780213 describes a two-stage process for preparing isocyanate-terminated siloxanes, in which amine-functionalized siloxanes are firstly reacted with carbonyl bisazolides to form siloxane-urea azolides. These are then pyrolyzed thermally to the corresponding isocyanate-terminated siloxanes. A disadvantage is the two-stage process which also requires expensive raw materials.